1. Field of the Invention
This invention relates generally to a roadside beacon system. More particularly, this invention relates to a roadside beacon system which is used to calibrate the position of a vehicle and to perform data transmission in a navigation system in which, after data representing a departure point are inputted, vehicle speed data and direction data are inputted to enable the display of the present position of the vehicle.
2. Background of the Invention
A so-called "navigation system" for vehicles has been known in the art. In the system, a small computer and a small display unit are installed on a vehicle. A road map is read out of memory means such as a compact disk and displayed on the display unit. On the other hand, the vehicle speed data outputted by a vehicle speed sensor and the direction data provided by a direction sensor are inputted, so that calculation of the position of the vehicle and determination of the traveling direction of the vehicle are performed at all times. According to the results of the calculation and the determination, the vehicle is marked on the road map displayed on the display unit.
With the navigation system, the operator in the vehicle can visually detect the present position and the traveling direction of his vehicle therefore, he can reach his destination without losing his way.
However, the navigation system described above is disadvantageous in the following point. In the system, the errors inherent in the vehicle speed sensor and the direction sensor are accumulated as the vehicle runs. When the distance traveled by the vehicle exceeds a predetermined value (which is not always constant, being determined by the errors of the vehicle speed sensor and the direction sensor of each vehicle and by the environmental conditions of the positions where the sensors are installed), then the position of the vehicle displayed on the display unit is greatly shifted from the true position. That is, the system becomes unreliable and the vehicle operator may lose his way.
In order to overcome this difficulty, a so-called "roadside beacon system" has been proposed. In the system, as shown in FIG. 7, roadside antennas 2 are installed at intervals shorter than the distance within which the accumulated error exceeds the above-described predetermined value. The roadside antennas 2 are used to transmit signals including position data and road direction data to respective predetermined relatively small areas (R shown in FIG. 4). On the other hand, the signals thus transmitted are received through a mobile antenna 4 installed on a vehicle 3 so that the position and the traveling direction of the vehicle are calibrated with a computer (cf. FIG. 7).
With the above roadside beacon system, the accumulated error is smaller than the predetermined value, and the position of the vehicle 3 can be displayed according to the correct position data and the accurate direction data at all times. This means the navigation system is reliable. If the roadside antenna is installed, for instance, near a railroad or a railroad crossing where the direction sensor is liable to erroneously operate, then errors attributed to external factors can be effectively eliminated.
In the above-described roadside beacon system, roadside antennas of considerably high directivity are used to transmit the aforementioned signals. The vehicles receive the signals only when passing through the areas converted by the signals. A conventional mobile antenna is sensitive mainly in a horizontal direction and has a wide directivity. Therefore, the mobile antenna 4 receives, as shown in FIG. 7, not only a signal component E directly from the road-side antenna 2 (hereinafter referred to as "a directly received signal component" but also signal components F, D and C which are reflected by a sound insulating wall 5, a road 1, another vehicle 3a, a buildings, etc. (hereinafter referred to as "indirectly received signal components").
Accordingly, the time-dependent strength distribution of the signal received by the mobile antenna is greatly different form the time-dependent strength distribution of the original signal transmitted through the roadside antenna. Thus, the conventional roadside beacon system suffers from a difficulty that the position and the traveling direction of the vehicle are calibrated according to the signal which greatly deviates from the original signal.
This problem will be described in more detail. When compared with the directly received signal component, the indirectly received signal components, reaching the mobile antenna through various paths, are different in phase and in amplitude. Therefore, depending on the phases, the indirectly received signal components are received as signals much larger or smaller in amplitude than the directly received signals.
Whenever the present position for the vehicle is required, the vehicle is traveling. As the vehicle runs, the aforementioned number of signal paths change and accordingly the signal received by the mobile antenna also changes irregularly with time, as shown in FIG. 8, thus causing a great error in the calibration.
The above-described phenomenon will be referred to as "a multi-path fading phenomenon".